Movie releases of a chemical kind

There are two pretty tense moments in 'Hunger Games: Mockinjay', and scientists know this just from the chemicals given off by the audience who watched the film. Dr Karl Kruszelnicki explains how a shared scary cinema experience led to a fundamental discovery about human biology.

Nowadays, thanks to the interwebs, you don't even have to go to the movies to watch a movie. You can see the latest ones at home on your TV, or even while travelling in a bus on your smartphone. But there are nice things about going out to the movies—mostly, the shared experience.

There was a very obvious jump in the isoprene level—corresponding exactly to when the heroine’s dress caught fire.

Speaking of shared experience, it turns out that, as a group, we humans release chemicals when something powerful happens on the screen. Sometimes, you can even tell what's happening on the screen by measuring the chemicals that the body discharges.

From an evolutionary point of view, this makes sense. Many creatures use chemicals, floating through the air, to communicate—plants talk to other plants, plants talk to insects, insects talk to each other, and there are many more examples.

But with humans, it's kind of tricky to verify this effect. On one hand, there are all kinds of claims for human airborne chemical communication.

It's said that there's a smell of fear and so you can communicate fear to another person, and that sweat odours can affect menstrual cycles, and that tears will affect testosterone levels, and that sleeping babies will respond to the smell of a lactating breast.

These claims sound reasonable. But on the other hand, there are very few large-scale and reliable studies backing up these claims.

Part of the reason is that the studies are hard to do. You need very sophisticated equipment to measure subtle changes in chemicals floating in the air. Luckily in this case, there already was a team of skilled scientists whose day job was flying over pristine Amazon forests, and sampling the volatile chemicals given off.

Now, most scientists love their work. So back in Mainz, Germany, during the Christmas holidays, the obvious thing to do was to keep working! Yes, the scientists wandered down to the local cinema, plugged in their fancy chemical analysis equipment, and measured the air coming from the audience. The air was pumped in under the seats of the audience, and out though vents in the ceiling.

Over several days, around Christmas 2013, they analysed the air from over 9,500 cinemagoers who watched 108 screenings of 16 different films. Over 870 volatile chemicals have been identified in human breath, but our scientists concentrated on just 100 of them.

And by the way, the film genres that produced the most recognisable and consistent chemical signatures were suspense and comedy. From the point of view of evolutionary biology, 'suspense' corresponds to 'be alert', while 'comedy' relates to 'stand down and relax'.

One of the chemicals that stood out was isoprene (C5H8). It has many roles in humans. This chemical is stored in your muscles, and is released when your muscles contract. But it's also involved with the production of cortisol— which is part of the flight-or-fight reaction. And just to show how complicated the human body is, it's also involved in the synthesis of cholesterol.

On each of four successive days, there was a peak in the isoprene level coming out of the movie theatre air-conditioning ducts when the audience stood up to leave the theatre— as you would expect. After all, they were using their muscles to stand up and walk out.

But the surprise was what happened during one of the movies, The Hunger Games: Mockingjay. At exactly 2:58pm, on each of the four successive days, there was a very obvious jump in the isoprene level—corresponding exactly to when the heroine's dress caught fire. And then there was another sudden jump at 3:07pm, when the final dramatic battle began. Perhaps, when the dress caught on fire and when the final big battle began, the audience tensed their muscles in unison, without even realising it.

Another chemical, carbon dioxide, also peaked. At both of these events in the movie action, exactly in synchrony with the jump in isoprene, there was also a jump in carbon dioxide. Maybe the audience, in addition to tensing up and becoming anxious, was also breathing faster.

There are so many fields where this new technology could be applied— psychology and biology, the making and marketing of films, and of course, in the diagnosis of disease.

But we might just have to hold our breath while the implications of this technology in other areas are explored.

However, I wonder if The Hunger Games: Mockingjay was as good for the scientists stuck outside the movie theatre, as it was for the audience inside?